This article shares tips on repairing IE if the browser has been hijacked (a term meaning that the user can no longer control the home page or aggressive pop-up activity). 

http://techrepublic.com.com/5100-6270_11-5034307.html

The key recommended steps are in this order (where you would take the “next step“ if a cleaning process still doesn't correct the underlying issues).

1. AntiVirus Cleaning -- Update your AV product to the latest scanning engine and virus definitions.  Scan your entire hard drive and remove items with your AV product or free cleaning tools available at many sites.

2. AdAware and SpyBot Search & Destroy -- Both of these free cleaning tools are excellent.  Use Google to find sites where you can download each product and update them to the latest available definitions each time you run. 

3. CWS Shredder, HijackThis, and Filemon/Regmon Analysis -- This next step requires a lot of technical expertise as Windows file and registry activity must be carefully analyzed. 

Dutch security expert Merjin has developed some great tools
http://www.spywareinfo.com/~merijn/
http://www.google.com/search?&q=merjin

I'm impressed with the security expertise of several moderators who analyze and share findings on how to correct IE hijacking issues. 

Bleeping Comuters - see HighjackThis Log Analysis forum
http://www.bleepingcomputer.com/forums/index.php

Sysinternals - Regmon / Filemon (monitors registry/file activity)
http://www.sysinternals.com/

4. Reinstallation of Internet Explorer -- As a last resort sometimes IE or even Windows itself will require reinstallation.  Always remember to apply critical patches before reconnecting to the Internet.  Then immediately perform a Windows Update to better protect your newly reinstalled environment.   

How to repair or reinstall Internet Explorer if needed

http://support.microsoft.com/default.aspx?scid=kb;en-us;318378